Some messages involved in communication exchanges among the stations of a distributed processing environment require an answer or reply message. For example, communication protocols adapted from International Standard ISO 4035, Data Communication High Level Data Link Control Procedures Elements of Procedures, first edition 1979-04-15, require that a link between two stations be in a data transfer state before data can be exchanged between the two stations. This is accomplished by a sending station transmitting to the receiving station a Set Asynchronous Response Load Command (SARM) command to the receiving station. The receiving station must return as a reply message an Unnumbered Acknowledgment (UA) to establish the link in a data transfer mode. The sending station can then begin transmitting to the receiving station information messages or I-frames.
The aforementioned High Level Data Link Control (HDLC) procedure provides a facility by which a sending station can demand that a receiving station return a reply message. This facility consists of a signaling bit contained in the message and called the Poll/Final (P/F) bit. This bit when set by the sending station requires (1) that the receiving station return a reply message with the P/F bit set and (2) that the sending station not issue any other frame on the link with the P/F bit set to one until it receives such reply message from the receiving station with the P/F bit set to one. A problem with such a procedure is that other processors could gain control of the communication bus or channel effectively locking out the reply message and thereby hanging up the message exchange for time periods on the order of several hundred milliseconds or more. This is unacceptable in distributed processing system environments.